The invention relates to an apparatus for reproducing digital information from a transmission medium, in particular an optically readable record carrier on which digital information is stored in the form of a track of optically detectable areas which alternate with intermediate areas. The apparatus generally comprises:
oscillator means for generating a clock signal, PA1 means for generating a decision level, and PA1 means for comparing the signal obtained from the transmission medium with the decision level in order to restore the digital signal.
Such apparatus is used in the "compact-disc digital audio" player which is commercially available from N. V. Philips' Gloeilampenfabrieken under the type designation CD 100. In this player a signal is generated which is a measure of the phase difference between the clock signal and the time base of the reproduced signal. The time base is determined by the intersections of the reproduced signal and the decision level, in order to lock the phase of the clock signal to said intersections by means of a phase-locked loop. This signal which is a measure of the phase difference is determined by measuring the time intervals between said intersections and the edges of the clock signal. When the reproduced signal is comparatively weak, for example owing to finger marks on the disc, this time measurement becomes very unreliable because it responds to intersections of the decision level caused by noise and the like. This gives rise to an erroneous phase detection and a consequent detuning of the clock oscillator. In order to mitigate these problems the phase detection is therefore interrupted in the event of the signal drop-out. However, this interruption demands some time, so that nevertheless the clock oscillator may be detuned.